Spiraling
Why should a football quarterback throw a
football with a spiral?
hunter-ed.com
Lots
of kinetic energy in a thrown football is lost
due to air drag. The force of air drag, which is
in the opposite direction of motion, is given by
½C(rho)(v^2)A.
The character we're interested in is A, the
area. When the football is pointed in the
direction it's moving, the ball will allow more
flow, a small A. This results in less
drag and thus can be thrown farther. In fact,
when a football is thrown well, with a perfect
spiral, it will only lose 41% of it's original
kinetic energy if the football is thrown at
pi/4 radians at 100ft/s. However, if a football
is thrown to 'tumble' through the air, drag is a
huge factor; 77% of the kinetic energy is lost if
the ball is thrown at
pi/4 radians at 100ft/s. The
difference in energy loss forces
quarterbacks to learn how to throw
the football with a spiral. This
concept is also applied to
bullets. When a gun fires a
bullet, the bullet spirals to the
target because the less air drag
will allow the bullet to go much
further/faster than if it was not
spiraling.
Do all football players use the spiral
technique when passing the football?
Sometimes, like for example during a kickoff,
a
kicker will purposefully
kick the football to tumble
'head over heals' to the
other side of the football
field instead of using a
spiral. The kicker does this
to increase A. While
the football is tumbling,
the football will have
significantly more air drag.
This is done in an attempt
to maximize air time to give
his team more time to run
down the field.